This invention relates to a method for manufacturing semiconductor devices each with an ion-implanted semiconductor substrate.
Hitherto, silicon oxide (SiO.sub.2) films have been used to separate semiconductor elements formed in a semiconductor substrate in case the substrate is made of silicon. As well known, however, positive charges exist at the interface between the silicon substrate and an SiO.sub.2 film. Due to the positive charge an inversion layer is formed in the substrate if the substrate is of p-type. This phenomenon is generally called "field inversion". Due to the field inversion a mutual electrical separation of the semiconductor elements becomes impossible.
To accomplish an effective separation among the semiconductor elements in a p-type substrate, it is therefore necessary not only to provide SiO.sub.2 films but also to elevate the acceptor concentration of the silicon substrate right beneath the SiO.sub.2 films to such extent that no inversion layer is formed in the substrate under each SiO.sub.2 film. Further those portions of the silicon substrate which are to become semiconductor elements should have such a acceptor concentration that inversion layers or channels can be formed by applying a suitable voltage to electrodes formed on said portions of the substrate through insulation films made of, for example, SiO.sub.2. This control of impurity concentration is carried out by ion injection technique.
There is known a method which meets the above-noted requirements and which can achieve a self alignment between a field oxide film and portions of a substrate with a high impurity concentration. Owing to the self alignment between a field oxide film and the portions having a high impurity concentration, the semiconductor chip need not have an excessive surface area to permit an erroneous masking. For this reason the known method manufactures semiconductor devices which can be made into highly integral IC's.
With reference to the accompanying FIGS. 1A and 1B it will be explained how to manufacture semiconductor devices in the above-mentioned known method. First, an Si.sub.3 N.sub.4 film 11 is deposited on a p-type silicon substrate 10. A photoresist 12 is then coated on that portion of the Si.sub.3 N.sub.4 film 11 under which a semiconductor element is to be formed. Using the photoresist 12 as a mask, the remaining portion of the Si.sub.3 N.sub.4 film 11 is etched to expose the upper surface of the substrate 10. This done, using the photoresist 12 as a mask again, boron ions are injected into the substrate 10, thereby forming a high impurity concentration layer 13 as shown in FIG. 1A. The layer 13 serves to prevent an inversion layer from being formed. Thereafter the photoresist 12 is removed from the surface of the Si.sub.3 N.sub.4 film 11. Using the Si.sub.3 N.sub.4 film 11 as a mask, the field region of the substrate 10, i.e. portion on which the high impurity concentration layer 13 is formed, is heated to form such a field oxide film 14 as shown in FIG. 1B.
As shown in FIG. 1B, however, the field silicon oxide film 14 also grows to the portion under the Si.sub.3 N.sub.4 film 11. As a result, the surface area of that portion of the substrate 10 which is to become a semiconductor element is made smaller than that defined by the photoresist 12. To control the ratio between the actual surface area and the surface area defined by the photoresist 12 is extremely difficult. A semiconductor element cannot therefore be made with a sufficient dimensional precision. In particular, it is difficult to form a small-sized element with a high dimensional precision. In view of this the known method is defective.
Further, a great stress concentrates on the edge of the field oxide film 14 when the film 14 is being formed. As a result, the silicon crystals in that portion of the substrate 10 which is to become a semiconductor element are mostly rendered defective. This would deteriorate the electrical characteristics of the resultant semiconductor element.
Moreover, since the field region of the substrate 10 is heated at a high temperature, i.e. 1,000.degree. C. for six to seven hours to form a field oxide film 14, the impurities in the high impurity concentration layer of the field region diffuse into that portion of the substrate 10 which is to become a semiconductor element. Consequently the electrical characteristics and operational speed of the resultant semiconductor element are deteriorated and lowered. The smaller is the semiconductor element, the more will be lowered its electrical characteristics and operational speed. In addition, the known method is rather complicated since it includes cumbersome steps such as deposition of an Si.sub.3 N.sub.4 film and etching process.